In a wide variety of applications, it is necessary to scan a laser beam along one or more directions, accurately and at high speed. For example, in laser printers, optical disc read/write systems, laser based displays, laser imaging, laser phototypesetting, fiber optic networking systems and a wide variety of other applications, such high speed and accurate scanning of the laser beam is critical. The conventional approach to scanning a laser beam for many such applications employs a rotating mirror to scan the laser beam in a linear direction as the mirror rotates. Typically, the mirror is configured in a polygon shape with each side corresponding to one scan length of the laser beam in the linear direction.
An example of such a rotating polygon laser beam scanner is illustrated in FIG. 1. The prior art laser beam scanning apparatus shown in FIG. 1 employs a polygon shaped mirror 1 which receives a laser beam provided by laser 2 and deflects the laser beam in a scanning direction X as the polygon 1 rotates. It will be readily appreciated from inspection of the geometry of FIG. 1 that such a rotating polygon system has the ability to scan the laser beam through a maximum angle of 180.degree. with a scan line duration determined by the rotational speed of the polygon divided by N, where N is the number of polygon sides. Also, it will be appreciated that for large N the scan angle may be significantly reduced below 180.degree.. Thus, for the eight sided polygon configured as illustrated in FIG. 1, the laser beam is scanned through an angle of about 90.degree. with the duration of each scan line being 1/8 the period for one rotation of the polygon.
The laser scanning apparatus illustrated in FIG. 1 has the advantage of being quite simple, and is suitable for some applications. Nonetheless, the inherent limitations of such apparatus make it impossible to simultaneously achieve a high degree of resolution, high scanning speed and a large scanning angle. More specifically, a high degree of resolution requires a relatively large polygon with few sides. That is, if the laser beam is to provide accurate information as it is scanned along the scan direction, modulation of the laser beam as it traverses the surface of the polygon side must unambiguously provide discrete points in the scan direction. Thus, each side of the polygon must increase with the beam diameter and the number of discrete scan points (n). Therefore, high resolution, corresponding to a very large number (n) of discrete scan points, in general requires large polygon sides. This limitation is particularly significant where the scanned beam target surface is located close to the polygon mirror. Also, as noted above, the scan angle is reduced as the number of polygon sides is increased. Therefore, high resolution and high scan angle require a large polygon with relatively few sides.
The requirements of a large polygon with few sides, however, mitigate against a high scan rate. As indicated above, scanning speed is directly related to the number of polygon sides. Therefore, a polygon with few sides requires very high speed rotation to achieve high scanning speed. Rotating a large polygon at high speed creates mechanical problems, however. In particular, high speed rotation introduces vibrations, stress on the moving parts, and reduced accuracy in the registration of the mirror relative to the laser beam. These factors collectively limit the rotational speed of the mirror, and hence the beam scan rate. Also, the requirement of a large polygon mirror can create severe space problems in compact applications such as laser printers, optical disc read/write devices and other applications where space considerations are important.
Accordingly, it will be appreciated that the conventional laser beam scanning apparatus illustrated in FIG. 1 has inherent limitations on its performance capabilities which make it unsuitable for applications where high speed scanning as well as high resolution and accuracy are required or where space limitations are present. A need thus presently exists for an improved laser beam scanning apparatus.